Mud motor pressure absorption tools

ABSTRACT

A pressure absorbing tool is attached to a workover motor in a milling assembly. The tool includes an upper housing portion that is secured to a drilling string, and a lower mandrel portion that is secured to the workover motor and is moveable axially, but not rotationally with respect to the upper housing portion between an axially compressed position and an axially extended position. Longitudinal grooves are inscribed on the lower mandrel portion, and a plurality of guide members are associated with the upper housing and disposed within the grooves. The axially retracting design uses pressure acting across two pistons to retract as the motor stalls. The Belleville washers bias the tool to the extended position once the motor has returned to operating conditions.

This application is a continuation-in-part of U.S. patent application Ser. No. 11/173,745 filed Jun. 30, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to pressure absorption devices that are used within a drill string during drilling operations.

2. Description of the Related Art

Traditionally, drilling of wellbores has been accomplished using drill bits that are affixed to the lower end of a drill string. The drill string is rotated in the hole to cause the bit to drill. As an alternative to traditional drill strings, drill bits are sometimes run in on a string of coiled tubing, which is run off of a spool located at the surface of the well. The coiled tubing is not rotated and, therefore, a downhole mud motor is used to rotate the drill bit at the lower end. Coiled tubing is less rigid than a traditional drilling string and, therefore, may be more vulnerable to damage from coiled tubing string drag, shock loads, and vibration effects on milling/cutting structures.

During drilling, the drill string is subjected to severe axial and torsional forces that can severely wear or damage components of the drilling string. Additionally, these forces can prevent the drill bit from maintaining good contact with the bottom of the borehole, thereby reducing the effectiveness of the drilling operation. Axial and torsional shock forces can significantly reduce the rate of penetration for a drilling tool. Torque generated by mud motors is proportional to the differential pressure.

A number of shock absorbing tools have been designed to absorb torsional and/or axial forces associated with drilling. However, most of these tools are primarily designed for use with rotary drilling strings. U.S. Pat. No. 6,543,556 issued to Anderson, for example, describes a torque and shock absorber for a traditional drill string wherein a mandrel is retained within a drive cylinder with a threaded or helical engagement between the two. Similar arrangements are found in U.S. Pat. Nos. 2,754,086; 4,443,206; 2,754,086, and 1,817,067.

A problem with prior art force absorbing arrangements is that the spiral interface used with the tool is often insufficiently robust to stand up to the rigors of a drilling environment. As a result, the tool will become inoperative. In an extremely undesirable situation, the mandrel may become canted or angularly slanted with respect to the upper housing due to the inadequate spiral interface. In this instance, the ability of the bit to drill is effectively destroyed, and the bit itself or other components may become damaged. Additionally, helically-retracting tools react to mud motor torque rather than to differential pressure.

The loads produced when running a mud motor at the bottom of a coiled tubing drilling string versus rotating the entire drilling string without a mud motor are similar, but different in some important ways. In workover motor applications, fluid is pumped through the string (threaded pipe or, more frequently, coiled tubing) to the motor. The coiled tubing drilling string is not rotated and, therefore, torque and speed are produced at the bottom of the well, rather than at the top of the well, and is resisted by the string above.

Particular problems are posed in milling operations where a workover motor is being used. In this particular situation, a milling tool is operated by a workover motor on coiled tubing. The milling environment creates problems such as erratic weight-on-bit (WOB) control due to coiled tubing string drag, vibration effects on milling/cutting structures, and efficiency issues associated with persistent stall outs. Additionally, as the workover motor reaches stall conditions, damage can occur to the mud motor and other bottom-hole assembly components.

The present invention addresses the problems of the prior art.

SUMMARY OF THE INVENTION

The invention provides devices and methods for absorbing pressure loads associated with drill strings that use drill motors, or mud motors, to operate the drill bit. In a particularly preferred embodiment, a pressure absorber is described that is particularly useful in milling operations wherein a milling tool is driven by a workover motor. Pressure spikes from the mud motor are absorbed by the pressure absorber, which react to differential pressure rather than mud motor torque. A preloading mechanism in the form of a compressible spring is used to maintain WOB over a given stroke length. The exemplary pressure absorption tool has a lower mandrel portion that is secured to the workover motor or associated component and an upper housing portion that is secured to the lower end of the coiled tubing or other drilling string accessories. The lower mandrel portion and the upper housing portion are operably interengaged by an interface that permits the lower mandrel portion to move axially, but not rotationally, with respect to the upper housing portion. The pressure absorbing tool provides improved operation due to use of guide members, such as guide pins or guide balls that engage axial grooves in the lower mandrel portion.

The preloading mechanism urges the tool to an axially extended position with the lower mandrel portion being extended outwardly from within the upper housing portion. In a currently preferred embodiment, a number of Belleville washers provide the spring force. Enlarged pistons actuated by motor differential pressure retract the tool. Belleville washers bias the tool to the extended position. The Belleville washers and pistons allow for a shorter, lighter-weight tool. Workover motor pressure spikes during stall conditions are absorbed, or at least reduced, as the tool moves to an axially compressed condition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side, cross-sectional view of an exemplary wellbore having a milling tool being operated by a workover motor on coiled tubing.

FIG. 2 is a side, cross-sectional view of an exemplary mud motor pressure absorption tool constructed in accordance with the present invention in an axially extended configuration.

FIG. 3 is a side, cross-sectional view of the mud motor pressure absorption tool is shown in FIG. 2, now in a compressed condition.

FIG. 4 is a cross-section taken along lines 4-4 in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic illustration of an exemplary wellbore 10 that is being drilled through the earth 12 by a drilling or milling system 14. The drilling system 14 includes coiled tubing 16 that is being unrolled from a spool 18 and disposed into the wellbore 10. An injector system 20, of a type known in the art for use with coiled tubing, is used to urge the coiled tubing 16 downwardly within the wellbore 10. A fluid pump 22 is associated with the coiled tubing 16 so as to selectively flow fluid into and through the coiled tubing 16.

The lower end of the coiled tubing 16 is typically secured to a motorhead assembly (MHA) (not shown), which is secured to a pressure absorbing tool 24, constructed in accordance with the present invention. The pressure absorbing tool 24 is, in turn, secured to a workover motor, or mud motor, 26 of a type known in the art for creating rotational motion under the impetus of fluid flowed axially through the motor 26. The motor 26 is secured to a milling tool 28. The milling tool 28 may be one of a variety of cutting tools used within a wellbore, including drill bits, underreamers, rotary mills and so forth. The motor 26 rotates the milling tool 28 with respect to the coiled tubing 16 in response to fluid that is pumped through the motor 26 by the pump 22.

FIGS. 2 and 3 depict the force absorbing tool 24 in greater detail. The tool 24 includes a top sub 30 with threaded connection end 32 for attachment to the coiled tubing 16. It is noted that the upper end of the tool 24 might also be secured to other drill string tools (not shown) or to threaded pipe (not shown). The top sub 30 is secured to an upper housing 34. The upper housing 34 encloses a radially enlarged spring chamber 36. Below the spring chamber 36, a reduced diameter sleeve 38 is defined within the upper housing 34. An interior stop shoulder 40 separates the chamber 36 and the sleeve 38. A pin housing 42 radially surrounds the lower end of the sleeve 38 and is secured, at its lower end, to a bottom sub 44 by set screws 46 and threading 48. The bottom sub 44 includes a threaded end connection 50 for interconnection of the tool 24 with the mud motor 26.

The spring chamber 36 contains a plurality of axially compressible Belleville washers 52 that surround an interior tubular guide sleeve 54. The Belleville washers 52 are stacked single or multi layers thick in an end-to-end, opposed relation, so that they are axially compressible. Although Belleville washers 52 are described herein, the tool 24 might, in fact, incorporate other compressive spring force mechanisms, such as gas or fluid springs, coil springs and so forth, as known in the art. A lower mandrel 58 is secured by threading 56 to the guide sleeve 54. The lower mandrel 58 includes an enlarged upper piston head 60 having annular elastomeric fluid seals 62, 63. A reduced diameter shaft 64 extends downwardly from the piston head 60 to a threaded end portion 66 that is affixed to the bottom sub 44. A lower enlarged piston head 67 is located below the piston head 60 and carries an annular elastomeric fluid seal 69. A slider portion 68 of the shaft 64 has multiple longitudinal grooves 70 inscribed thereupon. FIG. 4 illustrates four such longitudinal grooves located equidistantly about the circumference of the slider portion 68. However, there may be more or fewer than four such grooves. It is noted that within the lower mandrel 58, guide sleeve 54, and top and bottom subs 30, 44 is defined a central flowbore 72, through which drilling mud may be flowed.

A plurality of guide members, such as guide pins, or guide balls (pictured), 74 are securely retained within the upper housing 34 and are disposed to lie within the longitudinal grooves 70. As best shown in FIG. 4, the radial interior surfaces of the guide members 74 are shaped and sized to contact the grooves 70 in a complimentary manner. As best shown in FIG. 4, the guide members 74 preferably present a rounded, preferably hemispherical, inner contact surface 75 that materially eases movement of the guide members 74 within the grooves 70. Because the guide members 74 lie within the grooves 70, the lower mandrel 58 can move in a telescoping fashion with respect to the upper housing 34, without the housing 34 and mandrel 58 rotating with respect to one another. In a currently preferred embodiment, there are 16 such guide members 74. The guide members 74 are preferably distributed in a spaced relation about the entire circumference of the pin retainer sleeve 72, as FIGS. 2, 3, and 4 illustrate. This configuration provides for added stability of the lower mandrel 58 as it moves within the upper housing 34.

The lower mandrel 58, pin housing 42, bottom sub 44, and guide sleeve 54 collectively form a lower mandrel portion 81. The top sub 30, upper housing 34, and guide members 74 collectively form an upper housing portion 82.

During operation, the tool 24 is normally in the axially extended position shown in FIG. 2. The Belleville washers 52 and the balance of pressure across the two piston portions 60, 67 urge the lower mandrel 58 toward this extended position. As the workover motor 26 approaches stall conditions, the pressure increase acts upon both pistons 60, 67 to cause the tool 24 to lift the motor 26 and bit 28 off the bottom 71 of the wellbore 10, and the tool 24 is moved to the compressed position shown in FIG. 3. This reduces the weight-on-bit. In the compressed position, the upper end 78 of the housing 42 contacts a downward-facing exterior stop shoulder 80 on the upper housing 34. This contact limits the upward movement of the lower mandrel 58 with respect to the upper housing 34. Once the motor 26 has returned to its normal, non-stall operating condition, the Belleville washers 52 will return the tool 24 to the extended position, thereby restoring weight-on-bit. Extension of the lower mandrel 58 is limited with respect to the upper housing 34 by contact between the enlarged piston head 60 with the stop shoulder 40 (see FIG. 2). The pressure absorber tool 24 is designed to be run with workover motors to address the inherent challenges of milling operations. These challenges include issues such as erratic weight on bit (WOB) control due to coiled tubing string drag, shock, and vibration effects on milling/cutting structures, and efficiency issues associated with persistent stall outs. By reducing the motor stalls, the pressure absorber extends the useful life of the stator, increases the milling rate of penetration, and reduces trips in and out of the well to replace damaged motors. The pressure absorber tool 24 is run above a workover motor 26 and is designed to optimize WOB, absorb vibration and compensate for pressure spikes to minimize stalling during milling operations. The tool 24 incorporates a pre-load mechanism to maintain WOB over a given stroke length, and a means to convert pressure spikes associated with motor stalls to linear retraction.

Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof. 

1. A pressure absorbing tool for attachment to a workover motor in a milling assembly, the tool comprising: an upper housing portion that is secured to a drilling string; an lower mandrel portion that is secured to a mud motor; the lower mandrel portion being moveable axially, but not rotationally with respect to the upper housing portion between an axially compressed position and an axially extended position; and a compressive spring member to urge the lower mandrel portion toward the axially extended position.
 2. The tool of claim 1 further comprising: a longitudinal groove inscribed on the lower mandrel portion; and a guide member associated with the upper housing and slidingly disposed within the longitudinal groove.
 3. The tool of claim 2 wherein the guide member presents a rounded inner contact surface contacting the groove.
 4. The tool of claim 3 wherein the inner contact surface is hemispherical.
 5. The tool of claim 2 wherein there are multiple longitudinal grooves and multiple guide members.
 6. The tool of claim 1 wherein the compressive spring member comprises a Belleville washer.
 7. The tool of claim 6 wherein there are a plurality of Belleville washers, and wherein the tool further comprises a guide sleeve disposed radially within the Belleville washers.
 8. The tool of claim 2 wherein there is a plurality of longitudinal grooves inscribed on the lower mandrel portion.
 9. The tool of claim 8 wherein there are four longitudinal grooves inscribed on the lower mandrel portion.
 10. A milling system comprising: a drill string; a workover motor for operating a milling tool bit in response to flow of drilling fluid through the drill string; a pressure absorbing tool incorporated within the drill string above the workover motor for absorption of pressure spikes from the workover motor; the pressure absorbing tool comprising: an upper housing portion that is secured to the drill string; an lower mandrel portion that is secured to the workover motor; the lower mandrel portion being moveable axially, but not rotationally, with respect to the upper housing portion between an axially compressed position and an axially extended position; at least one longitudinal groove inscribed on the lower mandrel portion; at least one guide member associated with the upper housing and slidingly disposed within the at least one longitudinal groove; and a compressive spring mechanism to urge the lower mandrel portion toward the axially extended position.
 11. The drilling system of claim 10 wherein the drilling string comprises coiled tubing.
 12. The drilling system of claim 10 wherein the compressive spring mechanism comprises at least one Belleville washer.
 13. The drilling system of claim 10 wherein the compressive spring mechanism comprises a fluid spring.
 14. The drilling system of claim 10 wherein the compressive spring mechanism comprises a coil spring.
 15. The drilling system of claim 10 wherein the guide member presents a rounded inner contact surface contacting the at least one longitudinal groove.
 16. The drilling system of claim 10 wherein there are multiple longitudinal grooves.
 17. A method of absorbing pressure spikes associated with milling operations by a coiled tubing-run milling system, the method comprising the steps of: incorporating a pressure absorbing tool into the milling system above a workover motor; operating a milling tool of the milling system by flowing drilling fluid through the workover motor; and absorbing axial forces resulting from motor pressure spikes by moving an lower mandrel portion of the pressure absorbing tool axially, but not rotationally, with respect to an upper housing portion of the pressure absorbing tool, the lower mandrel further being axially guided by an interface of sliding of guide members within longitudinal grooves.
 18. The method of claim 17 further comprising the step of urging the pressure absorbing tool to an axially extended position with a compressive spring member and pistons. 